1. Field of the Invention
The present invention relates to a shake detecting apparatus and, more particularly, to a shake detecting apparatus that can be used as a device to be installed in a camera or other image shooting apparatus for the prevention of blurred pictures.
2. Description of the Related Art
Acceleration sensors, angular velocity sensors and rate gyros have conventionally been used for shake detecting apparatuses. Such shake detecting apparatuses are on the trend of down-sizing which is a current phenomenon widely observed in the field of electronic devices. However, as for shake detecting apparatuses, down-sizing can often result in a less accurate performance because output signals become more susceptible to drifts, offsets and other adverse effects that can be caused to by changes in the ambient temperature and/or the temperature of each apparatus itself. While the use of a piezoelectric device has been proposed as an effective means of down-sizing shake detecting apparatuses, such a device is particularly susceptible to drifts due to fluctuations not only in the physical dimensions but also in the capacitance that appear as a result of temperature changes.
FIG. 18 of the accompanying drawings schematically illustrates changes that can take place in the output signal of a shake gyro type angular velocity sensor in the initial stages of activation that operates as a shake detector. The horizontal and vertical axes respectively represent the elapsed time after energizing and the output level of the sensor. If the sensor is held stable, or not moved, its output signal changes rapidly for tens of several tens milliseconds (or 50 milliseconds in FIG. 18) after it is energized. During this period, the output rises quickly up to a little below its normal level (so-called "null voltage") if it is not subjected to a shake.
Thereafter, for several hundred milliseconds (for example, 300 milliseconds in FIG. 18), the output signal plunges into initial unstable stages, during which the output level mildly rises from a little below the null voltage to just the null voltage. When the initial unstable stages are over, the output level is totally stabilized as it moves into a stabilized normal phase of operation and does not show remarkable changes any longer.
In FIG. 18, the broken line indicates the output level of the sensor if it is subjected to a shake.
In a longer time span, however, the null voltage would change with time, or drift, although the output level is in the stabilized normal phase as a function of changes mainly in the ambient temperature.
While fluctuations of the null voltage attributable mainly to changes in the ambient temperature occur very slowly, probably a cyclic phenomenon with a period of several minutes, they still give rise to errors in the shake signal of the sensor. If the shake signal is used with errors due to drifts without taking any corrective measures, they remain in the signal and become amplified to immaturely and erroneously produce a saturated state in the sensor as the signal is amplified for use. Once a saturated state occurs in the sensor, it no longer can produce proper shake signals.
Japanese Patent Application KOKAI Publication No. 60-143330 discloses the use of a high-pass filter for removing low frequency components of the output signal of a shake gyro type angular velocity sensor. According to the cited document, the upper limit of frequency is specified for the high-pass filter so that it blocks any low frequency components of the signal below the specified level. The cut-off frequency and the time constant of a filter are inversely proportional. When a sensor and a filter to be used with the sensor are energized and if the upper limit of frequency for frequency cut-off specified for the filter is f, it takes a period of time equal to its time constant or 2.pi./f seconds before the output signal of the sensor is stabilized. An extra time period may have to be added to this time period to remove errors that may be given rise to by noises and other causes on the part of the sensor.
FIG. 20 shows a block diagram of a typical known arrangement for detecting a shake by using a shake gyro type angular velocity sensor. Referring to FIG. 20, the output signal of shake gyro type angular velocity sensor 1 is made to pass through high-pass filter (HPF) 2 that removes error components due to drifts and then amplified by amplifier 3 to a desired signal level. A voltage regulator 4 is arranged downstream relative to the amplifier 3 in order to cancel out the offset components of the signal generated by the amplifier 3 and make the output level of the signal get to the specified null voltage.
For detecting a shake of an image shooting apparatus produced by some external cause, it should be noted that the frequency of such a shake is low and about 15 Hz at most while the amplitude of such a low frequency vibration is typically large. Therefore, the use of a high-pass filter (HPF) having a large time constant is necessary to remove drifts in the output signal of the sensor without significantly attenuating the signal.
If a sensor of the type under consideration is used with a still camera, low frequency vibrations that may be applied to the camera cannot be removed by the photographer using the camera because he or she can hardly sense them instantaneously while taking a picture or depressing the shutter button. This is unlike the case of a video camera with which the operator of the camera can sense such vibrations as he or she watches the target through a view finder for a certain period of time. So, the net result will be an objectionably blurred picture. Means should be provided to avoid such a blurred picture by detecting low frequency vibrations and such means will be an HPF with a time constant of tens of several seconds.
FIG. 21 is a graph showing the relationship between the elapsed time after the start of operation of the arrangement of FIG. 20 and the output level of the HPF. As seen from FIG. 21, a considerable time period has to be provided until the output of the HPF is stabilized and the time period corresponds to the time constant of the filter. This is mainly because the input to the HPF remarkably changes in the initial stages of operation to reflect the abrupt change in the output of the sensor that occurs when it is energized.
Japanese Patent Application KOKAI Publication No. 63-50729 discloses an arrangement comprising an acceleration sensor and a plurality of integrators provided with respective HPFs with different time constants, wherein the integrators are selectively used for the operation of integration to be carried out for the acceleration sensor in order to minimize the time period of the initial unstable stages of the filter.
However, even with the arrangement of the above cited Japanese Patent Application KOKAI Publication No. 63-50729, the initial unstable stages last for a considerable period of time because the input to the HPF changes remarkably, reflecting the abrupt change in the output of the sensor that takes place when it is energized, although the arrangement is not without improvements.
Thus, the problem of drifts of a shake sensor is closely related with the time period required for the output of a shake detecting apparatus comprising such a sensor to become stabilized.
Points that have to be dealt with here are (1) the drift of the sensor and (2) the time period required for the HPF to remove the drift and stabilize its output.
For removing drifts by means of an HPF, using either of the techniques disclosed in the above cited Japanese Patent Application KOKAI Publications Nos. 60-143330 and 63-50729, the aggregate of the shake signal (angular velocity signal, acceleration signal) and the drift components needs to be found with a voltage range allowable for the input to the HPF. For instance, if a shake gyro type angular velocity sensor is used, drifts that are caused by temperature changes can be tens of several times as large as the angular velocity to be detected for the image shooting apparatus provided with the sensor may be involved. To cope with such drifts, the output of the sensor has to be amplified after passing through an HPF. However, an amplifier to be used for amplifying the output of the sensor can also be liable to generate drifts if they are small when compared with those of the sensor and offsets so that it may be necessary to arrange an additional HPF and/or a regulator in order to remove such offsets and drifts, making the entire circuit arrangement unacceptably complicated.
Another problem that needs to be solved for conventional shake detecting apparatus is that it takes a considerably period of time for the apparatus to determine if the output of the HPF has been stabilized or not because drifts are relatively large with regard to the effective signal level of the apparatus.
As a greater number of HPFs are used for a shake detecting apparatus, the entire circuit configuration of the apparatus inevitably becomes increasingly complex. While the operation of eliminating drifts can be carried out smoothly if a large number of HPFs with different time constants are used, such an arrangement can make the circuit configuration unacceptably complicated.
Additionally, while the above described technique of utilizing a plurality of HPFs involves the use of analog signals, the shake signal of the shake detecting apparatus representing the detected shake is processed digitally by means of a CPU (central processing unit) or a DSP (digital signal processor) after the completion of certain regulating operations such as the operation of regulating the reference voltage or the null voltage of the apparatus representing a no shake state of the image shooting apparatus that are required to be carried out before any shake signal is fed to the CPU or DSP by way of an A/D (analog/digital) converter. Without such regulating operations, the shake detecting apparatus does not work properly.
Such regulating operations are carried out typically by means of a trimmer resistor or a similar device before the shipment of the apparatus. Any errors involved in the regulating operations are directly reflected in the shake signal of the apparatus. If, for example, a shake correcting apparatus comprising shake detecting means is concerned, it can mistakenly operate to correct a non-existent shake if a series of regulating operations have not been carried out properly on the apparatus before the shipment.
Still another problem that needs to be solved for the conventional shake detecting apparatus is that, if the null voltage is shifted with time (due to drifts and/or offsets generated in the amplifying circuit of the apparatus) while the apparatus is in service, the shift cannot be corrected and the apparatus becomes inoperative. Note that such a shift of the null voltage can take place if the signal processing operation is an analog signal processing operation.
If the circuit of the shake detecting apparatus is so configured as to directly remove drifts out of the input fed from the sensor to a virtual HPF where a set of programs operate to remove drifts like as a real HPF does, the shake signal containing drift components therein has to be digitized, requiring the use of an A/D converter having a high resolution in the circuit when compared with the case where the shake signal containing no drift components. Again, such an arrangement makes the circuit complicated and requires a high degree of precision on the part of the A/D converter.
The apparatus may alternatively be so arranged that the output of the sensor of the apparatus is made to pass through an analog HPF and an amplifier before it is digitized and regulated again in a virtual HPF realized by a set of programs and operated by the CPU for additionally regulating any possible shift of the null voltage. With such an arrangement, however, the initial performance of the analog HPF remains unimproved.